2.6. Pressure‐flow relationships

When fetuses were 135 or 136 dGA and lambs were 5 or 6 dPNA, pressure‐flow relationship studies were performed to determine baseline and hyperemic coronary conductance and coronary flow reserve. Fetuses and lambs were given atropine and propranolol as above, and the left atrial catheter was pre‐filled with adenosine. A pressure‐flow relationship was determined by inflating either the inferior vena cava or post ductal aortic occluder with air over several seconds (inflation order was randomized between animals). This was repeated during adenosine‐mediated hyperemia using the adenosine rate determined from the dose–response study on the previous day (159 ± 68 μg min⁻¹ kg⁻¹). Each series of inflations was performed in at least triplicate. Example pressure‐flow data are shown in Figure 3a.

Pressure flow relationships assessed in fetuses (135 or 136 dGA, N = 7) and lambs (5 or 6 dPNA, N = 6). (a) Coronary pressure‐flow relationship examples from a single fetus (left) and a single neonate (right). Black points show baseline data and gray points show hyperemic data. Each point represents pressure‐flow data averaged over 1 s. (b) Coronary conductance at baseline (left panel) was lower in the neonate than the fetus (t‐test, p = 0.003), as it was with adenosine hyperemia (right panel, t‐test, p = 0.018). (c) Flow reserve was not significantly different between fetuses and neonates (t‐test p = 0.641). Columns are means, error bars + SD, dots are individual data. *p < 0.05.